The AM Forum

Hey guys,

Finally got my Ranger II on the air, and was thinking about a way to reduce the power to drive my SB-220.   I don't want to use an attenuator, and I don't want to jumper the plug and lose my grid current meter function.   How about if I install a VR to control the screen voltage of the 6146?  I was thinking then I could easily go back and forth from full power to a level suitable for driving a linear safely.   What are your thoughts?

Several methods for accomplishing control of output power level with the Ranger have appeared on this forum in the recent past. I am partial to use of a MOSFET, as described by WA3DSP.

Bob - NE

Thanks.  His method is exactly what I was talking about.  If I replace R37 with a pot, I can vary the drive to the clamper, which will in turn lower the screen voltage in the PA tube.   

  I did that on a Ranger and it works
very well, Output smoothly adjustable
from about 3 W to 25 or 30.

  I removed that knob from the crystal
cover and located the pot there. Very
Nice! Drill and tap the old knob for
a set screw and ur gud to go.. Also
had to break off a few of those spring
tabs that hold the cover on, 3 ea. top
and bottom to clear the bracket I made
to hold the pot.

GL

/Dan

Why do things the hard way when it's built in? Just turn down the drive pot to the appropriate RF output level and lower the mic gain as appropriate. Now guys, don't get all technical on me, that's what I did long ago and it worked just fine.

Turning the drive pot down reduces grid drive to the 6146 which is what you don't want to do. It will work but you put the 6146 in destruction mode and your audio generally sounds like crap.

I've tried that mod and while it does let you adjust the output power, it also increases audio distortion. The lower the output, the greater the distortion. Once you get to 8-12 watts, the transmitter sounds like crap. I'm not sure why some people think it's a good mod. Maybe if you are running unmodified stock audio the crud isn't as noticeable. ?? It's an easy mod to just JS for testing. Give it a try and let us know your opinion. As for me, I bought some inline Bird attenuators off epay. 9db (6+3)of attenuation is perfect for my Al-1500.

I'm not sure why reducing the screen voltage on the final would make the audio sound bad, as long as you give it the appropriate level.  Maybe I'm missing something.  I am fairly new.  I guess I could go with an attenuator, but I'm gonna give this a shot first.  If it fails, then I'll put a hot plate in line and do it that way.

lowering the screen voltage to lower output power raises the impedance that the tube is operating at .... this can be dangerous to the health of the modulation transformer due to unexpected high voltage generation due to it being unloaded .... it is far better to reduce both B+ and plate current to a value that will give you the same modulated impedance as the stock unit ... for example

stock:B+ = 650V   plate current = .12A     Zmod = Ep / Ip  = 650/.12 = 5.4 kOhm  Prf out = 50W

modified: B+ = 350V  plate currrent = .065A    Zmod = 5.4kOhm    Prf out = 16W

keep the modulator tubes on original 650V B+ .... this gives mucho audio reserve and cleaner audio
no changes should be necessary to the rf side since the rf impedance would not be changed... you can use a 2E26 tube for a final if you wish

if you insist on doing the screen vari-power modification at least add negative cycle loading with a diode and power resistor to help protect yer mod xfmr ...this is a part of the infamous 3 diode limiter ....gl

If one were to put a variable resistor in series with the HV to the secondary of the mod transformer to
vary the HV from stock (650) to as low as 200 volts, what would be the value of the resistor including dissipation?
Inquiring minds want to know.
Thanks,
Don W9BHI

ok dropping 450V but what is the plate current ?  I am guessing 30 to 35 mA for the sake of discussion:

R = E/I  =  450 / .030  =  15 kOhm   P = IE  =  13.5W   ymmv

you should use a coupling capacitor across this dropping resistor

OK sounds good to me.
Why a coupling cap across the resistor?
It will be on the PS side of the mod secondary.
Maybe a 100 mfd to ground at the modulation transformer and resistor junction for an AC ground?

Don W9BHI

I think I'll just build a pad attenuator.  ;D  Basically a copy of the 6db one that Johnson sold for use with the thunderbolt.  Yeah, I know.  Long way 'round to the original answer, but I'm not quite sure I'm comfortable with modifying the HV.  I'm not sure I can even find a pot rated for that that will fit behind the crystal socket cover. 

     The best way I have found to drop the power of a plate modulated transmitter to drive an amp is to insert a 25 watt or so wirewound rheostat (about 5 K is what I used) with a nonpolarized electrolytic or oil cap across it in series with the modulated B+. This was suggested to me by Tim WA1HLR. I used it on my ARC-5 setup and it works perfectly. The other methods I have used are: Lightly load the PA, or drop the screen voltage. Both resulted in non linear modulation and various forms of distortion. Lowering the drive level pot is out of the question, as you will not be running the PA in class C. The only drawback to the series resistor idea is that the PA screen must get it's voltage from a screen resistor off the modulated B+, which may not be the case in some transmitters. As a bonus you get very clean audio with tons of headroom. Limiting and/or compression of the audio is also a must.
     
     BY using an RF attenuator and running the Ranger at normal power you will still get the distortion from the saturation of the tiny Ranger mod transformer and stock audio, the splatter from which will then be amplified to make folks near you miserable. You need to drive the amp with a clean signal. 

Ugh.... My head hurts.  I want to go home!   :P  I'll give all of this some thought.  I like the idea of the extra headroom for sure.  And I definitely don't want to mess up the transmitted signal / audio, which according to all I've spoken to is great right now, and a couple of them were not just listening, but also looking at my signal.    I'll have to pull up the schem for the ranger II and give the PA section a good onceover.  In the meantime, I can just get back to work on other projects, like re-housing the inards of this SB-220 into a proper cabinet with the PS separate, and blower cooling with chimneys.  Once that's done, continuous carrier service should be greatly increased anyway. 

the coupling cap is to keep from developing audio power across the resistor and discarding it as more heat as well as presenting the correct impedance load to the modulator .... this way you can run the modulator at lower power where it is cleaner ....JBL is right about using something to control peaks

The dropping resistor works fine business.

This is the same method Johnson used to make the CB radios out of ham chassis (mobiles).

Johnson Messenger line for schematics / ideas.   

--Shane
KD6VXI

Ok.  I think I like it.  Looking at the schematic for the messenger 1, I see how they did it.  I'm thinking a 15K should get it down to 150 volts or so when maxed.  By the way, have you guys seen how much those rheostats cost!?! Jeebus!  :o  Anybody got one in the junk drawer?   :P

I'm going through reostat  shock myself.   Wanted to add one to a tube with handles project for fil voltage.....   Was cheaper to buy a variac off the bay....

--Shane
KD6VXI

Managed to get one off the bay for 15 clams shipped...

Has anyone ever substituted a 2E26 for the 6146???  Seems like a simple fix to cut the power down.....I have never tried it....tell me why it won't work....Steve

According to TimTron, and others, the plate impedance will be too low on 75 and 160 due to the tank design.  After a ton of reading on the subject, reduction of the plate voltage via a wirewoumd resistor or rheostat is the easiest on all involved components.  All that will be required is being careful with the peaks on modulation. 

So the rheostat arrived, and, after talking with timtron via a couple of super helpful emails, I installed it in series between pin 2 and pin 6 of the accessory plug with a short piece of heavy zip cord.  Just for testing.  I've got a project box for it.  With the rheostat there I was able to lower the B+ to the secondary of the MT, which gave me variable output.  I was able to set the rig up perfectly for driving the SB-220 to any carrier level i wanted.   As expected, I had to cut back on the audio, but once that was dialed in, everything was great.  No flat topping, no extra garbage on the spectrum that I could see.   The rheostat gets hot, but not dangerously so.  Pretty satisfied with this method for now.  Gonna do the audio mods next week.  Thanks for the time, guys.